The invention relates to a device for actuating an electromagnet as it used in safety or security circuits which themselves are used in connection with safety switches for surveying the open and closed positions of doors, flaps or the like of restricted areas in production lines or the like.
In such safety or security circuits for locking and unlocking doors, flaps or the like, electromagnets are used which have an armature which can be activated in accordance with the actuation of a solenoid and which, on the basis of its position, locks or unlocks a door. It must be ensured here that the armature moves correctly during a pull-in section and that a magnetic force which is exerted by the solenoid for holding the armature in its position during a holding section when there is the lowest possible operating voltage applied is not reduced to such an extent that the armature drops out as a result of external faults, in particular vibrations.
A predetermined application of voltage to the solenoid does not, however, allow for the current flowing through the solenoid to be monitored in terms of an energy loss or the operating temperature of the solenoid so that it is advantageous to interrupt the application of voltage when the solenoid heats up and/or when specific current values are reached, in order to prevent energy loss through a generation of heat by the solenoid.
Published German Patent Application No. DE 43 41 797 A1 discloses that a current flowing through an electromagnet load, for example in the form of a solenoid, is limited by a current regulating means to a predetermined value which is higher in an pull-in section than in a holding section. For this purpose, a switch is used with which the solenoid can be temporarily disconnected from the voltage applied to it if the respective current value is reached. The switch is closed again if a respective low current value is reached. In order to determine the current flowing through the solenoid, the device uses a measuring device which is connected to a current evaluation means. The current measured by the current evaluation means is compared with a maximum current by a current regulator, the current regulator generating an actuation signal which is applied to an output stage which itself actuates the switch. This is costly owing to the use of numerous different components and has, in particular with the current evaluation means and the output stage, multi-component parts.
German Patent No. DE 195 22 582 C2 discloses that, to actuate an electromagnet which activates an armature, a voltage is applied to a solenoid with a predetermined periodicity. A period has sections of time with different lengths. It is possible to distinguish between pull-in sections and relatively long holding sections, the pull-in sections having essentially a long pulse and the holding sections having a plurality of shorter pulses for applying voltage to the solenoid. The pull-in sections serve the purpose of pulling in the solenoid, a larger current being present at the end of an pull-in sectionxe2x80x94owing to the longer pulses in comparison with the pulses during the holding sectionsxe2x80x94than flows through the solenoid at the end of a pulse of the holding sections. The holding sections have the purpose of maintaining a relatively small current which flows through the solenoid and which is sufficient to hold the armature in its position.
It is an object of the invention to provide a device for actuating an electromagnet which uses simple components and is of simple design.
The invention concerns a device for actuating an electromagnet, in particular for a safety circuit, which has an armature which is activated in accordance with the actuation of a solenoid; comprising
a switch which is connected in series to the solenoid and via which a d.c. voltage is applicable to the solenoid by closing the switch;
a device for temporarily interrupting the application of the voltage as long as a predetermined current flowing through the solenoid is detected;
a first clock generator for actuating the switch in a predetermined first clock cycle;
a first and a second resistor, which are each connected in series to the solenoid and in parallel with one another;
a current detection switch, connected in series to the switch, for detecting the current flowing through the solenoid;
wherein the magnitude of the first resistor which is connected in parallel with the current detection switch determining, in conjunction with the response threshold of the current detection switch, the magnitude of the holding current for the armature; and
wherein the second resistor is connectable into the circuit for generating an attraction current for the armature, via a further, second clock generator in a predetermined second clock cycle, which at least partially overlaps with the first clock cycle.
The device for actuating an electromagnet is provided in which a d.c. voltage applicable to a solenoid periodically in the clock cycle by means of clocked closing of a switch, it being possible to interrupt the application of the d.c. voltage when a predetermined current value is reached, by means of a current detection switch through which a partial current flowing through the solenoid flows as a function of a first resistor which is connected in parallel with the current detector switch and in series with the solenoid. A different maximum current in pull-in sections and holding sections is achieved here by virtue of the fact that, in a second clock cycle, which overlaps at least with the first clock cycle, a second resistor can be connected into the circuit in parallel with the first resistor in order to generate an attraction current. In this case, a relatively high current can flow through the solenoid as, as a result of the connection into the circuit of the second resistor, the magnitude of the resulting relatively small overall resistance determines the magnitude of the current flowing through the solenoid. As a result, a very simple design can be achieved using simple components.
Further objects, advantages and embodiments of the invention can be found in the following description.